[8.13] Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197

J. P. Halpern, E. V. Gotthelf (Columbia U.)

Two observations of the 5.54~s Transient Anomalous X-ray
Pulsar XTE J1810-197 obtained 6 months apart with
XMM-Newton are used to study its spectrum and pulsed light
curve as the source fades from outburst. The decay is
consistent with an exponential of time constant \approx
300 days, but not a power law as predicted in some models
of sudden deep crustal heating events. Both spectra are well
fitted by a model that commonly describes anomalous X-ray
pulsars (AXPs), an absorbed blackbody plus a steep power
law. However, a two-temperature blackbody fit is also
acceptable, and better motivated physically in view of the
faint optical/IR fluxes, the X-ray pulse shapes that hardly
depend on energy in XTE J1810-197, and the inferred
emitting areas that are less than or equal to the surface
area of a neutron star. The fitted temperatures remained the
same while the flux declined by 46%, which can be
interpreted as a decrease in area of the emitting regions.
The pulsar continues to spin down, albeit at a reduced rate
of (5.1 ±1.6)\times10-12 s~s-1. The inferred
characteristic age \tau\rm c \equiv P/2\dot P \approx
17,000~yr, magnetic field strength B\rm s \approx
1.7\times 1014~G, and outburst properties are consistent
with both the outburst and quiescent X-ray luminosities
being powered by magnetic field decay, i.e., XTE J1810-197
is a magnetar. Continuing study of XTE J1810-197 in
various states of luminosity is important for understanding
the range of thermal and non-thermal emission mechanisms to
which magnetars convert their energy, and for possibly
unifying the growing classes of isolated, young neutron
stars that are not powered by rotation.